1. Field of the Invention
The present invention is a device to modify isotropic gamma radiation fluxes so that doses of radiation received by an irradiated product are uniform. More specifically, the present invention is a device placed between a radiation source and a product being irradiated for reducing (but not eliminating) the number of photons that are not traveling at or near desired angles (e.g. right angles) to the product's face plane, without significantly reducing those approaching the target's face plane at desired angles (e.g. right angles).
When products are irradiated by gamma radiation to produce a beneficial chemical, physical or biological effect, a certain amount of undesirable nonuniformity results. These nonuniformities result from four fundamental factors:
1. the geometry of the radiation sources and of the product, and their geometric relationship to one another;
2. the isotropic nature of the radiation emitted by the radioactive isotope sources;
3. the mass attenuation factors of the materials being irradiated; and
4. the average bulk densities of the materials being irradiated (specific mass involved).
The problem can perhaps best be understood by describing it as "surface burn". The surface of the product being irradiated is overdosed compared to the interior of the product, much the same way that a roast turning on a rotisserie can be burned on the surface while the interior is still raw.
When a product is being irradiated to achieve a specific purpose, it is necessary to make sure that all parts of the product receive at least the amount of radiation required to accomplish the effect desired. This amount of radiation is designated the MINIMUM DOSE (Dmin).
In some cases, however, too much radiation received by the product can produce an undesirable result (product damage) or the dose may exceed a mandatory regulatory agency limit and become "legally tainted". This amount of radiation is called the MAXIMUM RADIATION DOSE (Dmax).
Obviously irradiators are designed to deliver a dose of radiation to all parts of a product that is within these limits (&gt;Dmin, but &lt;Dmax). Unfortunately, to achieve this goal, it has heretofore been necessary to sacrifice the efficiency of the irradiator or of the operations, or both. The two traditional methods to reduce nonuniformity (reduce Dmax/Dmin) is to irradiate "thinner" layers of the product, or increase the distance between the radiation source and the product. The first method sacrifices operational efficiency (increases material product handling), while the second reduces Radiation Utilization Efficiency (the percentage of the radiation usefully absorbed in the product compared to the total amount emitted by the radiation source). The analogy of the roast on the rotisserie still holds; either the roast must be cut into thinner pieces and cooked separately, or moved farther from the flame and thus take longer to cook.
The specific reason for "surface burn" is due to the isotropic nature of the radiation emitted by radioactive isotopes and the "inverse square" phenomenon that results. All radiation (photons) in the electromagnetic spectrum behave in this manner, including visible light.